ovsdb-idl.at: Wait all servers to join the cluster.

ovsdb-idl.at: Wait all servers to join the cluster.

The test 'Check Python IDL reconnects to leader - Python3
(leader only)' fails sometimes when the first ovsdb-server
gets killed before the others had joined the cluster.

Fix the function ovsdb_cluster_start_idltest to wait them
to join the cluster.

Fixes: c39751e44539 ("python: Monitor Database table to manage lifecycle of IDL client.")
Co-authored-by:: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>